The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Moving Picture Experts Group (MPEG) and Video Coding Experts Group (VCEG) have developed video compression technologies that are more excellent than and superior to the existing MPEG-4 Part 2 and H.263 standard. This new standard is called H.264/AVC (Advanced Video Coding) and was jointly announced as MPEG-4 Part 10 AVC and ITU-T Recommendation H.264.
H.264/AVC (hereinafter, referred to as ‘H.264’) utilizes a spatial prediction coding method different from the existing international video coding standards, such as MPEG-1, MPEG-2, MPEG-4 Part 2 Visual, and the like. The conventional method utilizes “intra prediction” for coefficients transformed in a DCT transform domain so as to increase the coding efficiency, resulting in degradation in subjective picture quality at a low-band transmission bit rate. However, H.264 adopts a spatial intra prediction encoding method in a spatial domain, instead of a transform domain.
According to the spatial intra prediction in terms of a video encoder, information on a block to be currently encoded is predicted from information of previously encoded and reproduced block, and only difference information of actual block information to be encoded is encoded and transmitted to a video decoder. In this case, the prediction may also be performed by transmitting prediction direction information necessary for prediction to the video decoder, or synchronizing the video encoder and the video decoder. In terms of the video decoder, information of the block to be currently decoded is predicted by using information of previously decoded and reproduced peripheral (or neighboring) blocks, and desired configuration information is reproduced by obtaining the sum of the predicted block information and the difference information transmitted from the video encoder. Even in this case, when prediction direction information required for prediction is transmitted, the corresponding prediction direction information is used after decoding.
H.264 performs encoding or decoding in units of macroblocks of 16×16 pixel block size by using either of intra prediction and inter prediction. Examples of the intra prediction according to H.264 standard include intra 4×4 prediction, intra 8×8 prediction, and intra 16×16 prediction, each of which includes a plurality of prediction modes. In addition, the intra prediction generates predicted blocks by using previously encoded or decoded pixels located directly above and on the left of a block to be encoded or decoded.
In the case of the intra 4×4 prediction, as shown in FIG. 1, nine prediction modes are defined in consideration of positions and prediction directions of peripheral (or neighboring) pixels. The nine prediction modes include a vertical mode (mode 0), a horizontal mode (mode 1), a direct current (DC) mode (mode 2), a diagonal down-left mode (mode 3), a diagonal down-right mode (mode 4), a vertical-right mode (mode 5), a horizontal-down mode (mode 6), a vertical-left mode (mode 7), and a horizontal-up mode (mode 8).
In the case of the intra 8×8 prediction, the nine prediction directions shown in FIG. 1 are used as in the case of the intra 4×4 prediction, and a method of calculating the predicted pixels is substantially identical to the intra prediction encoding based on units of 4×4 pixels, except for a difference in block size (4×4 pixels against 8×8 pixels).
Furthermore, in the case of the intra 16×16 prediction, four prediction modes are defined as including a vertical mode (mode 0), a horizontal mode (mode 1), a DC mode (mode 2), and a plane mode (mode 3), as shown in FIG. 2.
Referring to FIGS. 1 and 2, peripheral pixels used for prediction are limited to the left and top. This is because only pixels reconstructed after the decoding process are usable for prediction. The encoder encodes blocks from left to right and from top to bottom, depending on a coding order, and a bitstream resulting from block compression is also transmitted sequentially to the decoder. Therefore, reconstructed pixels exist on the left and top of the block to be currently encoded. However, since peripheral pixels usable for the intra prediction are limited to the left and top, spatial redundancy may not be effectively reduced in the intra prediction mode, which is a main factor to lower compression efficiency.
Meanwhile, in H.264 standard, the intra 8×8 prediction or the intra 4×4 prediction within the macroblock determines a block coding order according to a raster scan method as shown in FIG. 3. The raster scan method is suitable in the case of considering only pixels located on the left and top of the current block to be encoded or decoded, as shown in FIGS. 1 and 2. However, the raster scan method has a problem that cannot consider previously encoded or decoded pixels located on the right and bottom among the peripheral pixels of the current block.
In particular, in order to efficiently compress a high-resolution video such as a 4K×2K video, research has recently been conducted on an encoding/decoding method using a macroblock having a size larger than a 16×16 pixel-block (hereinafter, referred to as an extended macroblock for the sake of convenience). Even with the use of such extended macroblock, the encoding/decoding operation has been performed by only making divisions in units of 16×16 pixel-blocks as in the H.264 method. For example, assuming that the size of the extended macroblock is a 32×32 pixel-block, a syntax structure in the case of using the extended macroblock is provided as shown in FIG. 4. Also, since four 16×16 macroblocks exists in the extended macroblock, data of 16×16 macroblock units are contained in data of the extended macroblock. In this case, when the encoding/decoding is performed in units of 16×16 macroblocks, a single macroblock may use only one of an intra prediction mode and an inter prediction mode. However, in order for efficiently encoding/decoding high-resolution videos, it is necessary to perform the encoding/decoding in units of extended coding blocks. In the case where large-sized extended coding blocks, such as 32×32 pixel-blocks or 64×64 pixel-blocks, are used as encoding/decoding units, statistical features different from the existing encoding/decoding methods are variously generated. For example, when there are signals having a lot of motions or a spatially complicated type in high-resolution videos, compression efficiency can be improved when blocks inside the extended coding block are encoded/decoded only through a combination of inter and intra modes. If the existing video encoding/decoding method is used, the encoding/decoding efficiency is lowered and the picture quality is degraded.
In addition, as described above, the H.264 intra prediction generates predicted blocks by using previously encoded or decoded pixels located directly on the top and on the left of the block to be encoded or decoded. In the case where the video encoding/decoding is performed in units of extended coding blocks, when videos are encoded or decoded in the combined inter and intra modes in the extended coding blocks, the existing intra prediction method may not utilize pixels located on the bottom of the current block to be encoded or decoded and pixels located on the right thereof. Consequently, it may be difficult to increase the compression efficiency.